The present invention relates generally to magnetic resonance imaging systems, such as those used for medical diagnostic applications. More particularly, the invention relates to a technique for shielding radio frequency magnetic fields in MRI systems via an adapted gradient field coil which serves to produce the desired gradient fields, while shielding RF energy.
Magnetic resonance imaging (MRI) systems have become ubiquitous in the field of medical diagnostics. Over the past decades, improved techniques for MRI examinations have been developed that now permit very high quality images to be produced in a relatively short time. As a result, diagnostic images with varying degrees of resolution are available to the radiologist that can be adapted to particular diagnostic applications.
In general, MRI examinations are based on the interactions among a primary magnetic field, a radio frequency (RF) magnetic field and time varying magnetic field gradients with nuclear spins within the subject of interest. The nuclear spins, such as hydrogen nuclei in water molecules, have characteristic behaviors in response to external magnetic fields. The precession of such nuclear spins can be influenced by manipulation of the fields to obtain RF signals that can be detected, processed, and used to reconstruct a useful image.
The magnetic fields used to produce images in MRI systems include a highly uniform, primary magnetic field that is produced by a magnet. A series of gradient fields are produced by a set of three coils disposed around the subject. The gradient fields encode positions of individual volume elements or voxels in three dimensions.
A radio frequency coil is employed to produce an RF magnetic field. This RF magnetic field perturbs the spin system from its equilibrium direction, causing the spins to precess around the axis of their equilibrium magnetization. During this precession, radio frequency fields are emitted by the spins and are detected by either the same transmitting RF coil, or by a separate receive-only coil. These signals are amplified, filtered, and digitized. The digitized signals are then processed using one of several possible reconstruction algorithms to form a final image.
Many specific techniques have been developed to acquire MR images for a variety of applications. One major difference among these techniques is in the way gradient pulses and RF pulses are used to manipulate the spin systems to yield different image contrasts, signal-to-noise ratios, and resolutions. Graphically, such techniques are illustrated as xe2x80x9cpulse sequencesxe2x80x9d in which the pulses are represented along with temporal relationships among them. In recent years, pulse sequences have been developed which permit extremely rapid acquisition of a large amount of raw data. Such pulse sequences permit significant reduction in the time required to perform the examinations. Time reductions are particularly important for acquiring high resolution images, as well as for suppressing motion effects and reducing the discomfort of patients in the examination process.
A difficulty which arises in MRI systems involves the interaction between the RF magnetic field and the surrounding gradient coil structures. In particular, the RF magnetic field can penetrate into the gradient coil structures and, due to the very lossy nature of these structures, can be dissipated if not otherwise shielded. The loss of RF energy can result in the need to boost input levels to the RF coil to obtain the desired field strength. However, higher energy levels imply higher powered amplifiers used to drive the RF coil, and can lead to excessive energy levels which are undesireable within the patient bore of the scanner.
Heretofore known techniques for limiting RF energy loss in MRI systems have included various shield configurations placed within the gradient coil assembly. The RF shields prevent or considerably reduce penetration of the RF magnetic field into the gradient coil assemblies, thereby reducing RF energy losses. However, the placement of RF shields within the gradient coil assembly results in relatively close proximity between the RF shield and the RF coil. Such placement reduces the effectiveness of the coil somewhat, as well as the efficiency of the gradient coils, which must be placed radially beyond the RF shield.
There is a need, therefore, for an improved technique for shielding RF magnetic fields in MRI systems. There is a particular need, at present, for a technique which will limit energy losses within the gradient coils, while enhancing the efficiency of both the RF shield and the gradient coils by judicious placement of the shielding structure with respect to the patient bore and RF coil.
The present invention provides a novel technique for RF shielding in MRI systems designed to respond to these needs. The technique may be employed in new systems, but may also be retrofitted to existing systems where desired. The technique offers a combined gradient coil/RF shield structure which reduces or eliminates the need for a separate RF shield. In one embodiment, an inner gradient coil, such as a Z-axis coil which is inherently decoupled from the RF magnetic field due to the orientation of its field, is placed at an innermost location within the gradient coil assembly. One of the remaining two gradient coils, preferably the gradient coil adjacent to the Z-axis coil, is adapted to provide shielding at the RF frequencies, while still performing its functions in producing the desired gradient fields.
The combination gradient coil/RF shield includes a conductor or conductors supported on a support structure, such as a non-conductive tube. The gradient coil conductor is rendered reflective of energy at the radio frequencies by capacitors linked between the conductive paths formed by the conductor. The circuit thus defined by the capacitors and conductor serves as a shield at radio frequencies, while allowing gradient fields at the much lower frequencies of gradient coil operation to be formed in a conventional manner.